The most likely reason for this is the actuals thru date on the task assignment is being updated. If the task assignment has ETC's this can affect the task dates. This can occur in the following scenario:
Note: Even if the timesheet that is posted does not contain any entries for the task, this issue can occur. The reason being is PPM will see that the resource posted time thru a specific date. As the task still has ETC on it, PPM will adjust the dates accordingly as it considers the task still open due to ETC remaining.
Workaround: The one option to workaround this behavior is to close the project for time entry by unchecking 'Open for Time Entry' at the project level.